The present invention relates to the electronic amplifier art and, in particular, to an amplifier which includes means to vary the amplifier's bias level as a function of input signal level and load impedance.
A critical link in modern high fidelity reproduction systems is the audio amplifier. In a typical installation, the audio amplifier connects between a signal source, such as the output from a compact disc player, and the loudspeaker. The amplifier both voltage and power multiplies the input signal in order to drive the loudspeaker to a satisfactory level.
It has been found that listeners can discern various forms of amplifier distortion which, although a small percentage of the reproduced signal, nonetheless constitutes a fatigue inducing irritation. One source of such distortion is clipping induced by input signal levels which drive the amplifier beyond its linear drive region and into saturation of the output power devices. A common approach to combat clipping is to provide very high power amplifiers which are capable of large signal excursions before the output devices are driven to saturation. Such high power amplifiers are expensive to manufacture due to the high cost of high voltage, high power semiconductors and the need for expensive power supply components.
To minimize the internal heating of output devices in high power amplifiers, the output devices are commonly operated in a class AB mode. A necessary limitation of class AB operation is the production of cross-over or notch distortion as symmetrical push-pull devices make the transition between conducting states. While such notch distortions may be a small percentage of the total signal supplied by the amplifier, it is nonetheless objectionable and fatiguing to listeners. From the listener's standpoint, class A amplifier operation is much preferred in that the above-described cross-over distortion is eliminated. However, class A mode operation results in unacceptably high power dissipation in the output power devices, thereby requiring many such devices with large attendant heat sinks, fans, and so forth.
Thus, there has been a long felt need in the audio amplifier art for an amplifier design that exhibits class A sound quality with class AB device dissipation.
On such solution known to the prior art is described in U.S. Pat. No. 4,638,260 which issued Jan. 20, 1987 and is assigned to the same inventor as the present application. Described therein is a bias control circuit which combines a static bias signal with a dynamic bias signal to produce a resultant bias signal which, when coupled through an optical-coupler, dynamically biases an otherwise AB amplifier into a class A amplifier under dynamic signal conditions.
While the above-referenced invention defines a distinct advance in the audio amplifier art, it does not take into account variations in the impedance of the load. The impedance of a loudspeaker is specified by its manufacturer, with common values being 2, 4, 6 or 8 ohms. These stated impedance values are nominal, and as a practical matter in a given loudspeaker design the impedance can vary considerably as a function of frequency. For example, it is not uncommon to find that a loudspeaker which is rated at a nominal 8 ohm impedance exhibits 40 ohms or more at its resonance point. Since power equals the square of current times resistance, an amplifier must develop 1 amp into an 8 ohm load to produce 8 watts of power. On the other hand, the same amplifier must deliver 2 amps of current into a 2 ohm load in order to produce 8 watts of power. In order to assure class A mode operation of the output power devices, they must always pass slightly more current than required by the load to thereby assure that the devices never shut off. In the example above, the output devices need only be biased at a current greater than 1 amp when driving an 8 ohm load to produce 8 watts or less of class A operation, whereas into a 2 ohm load they must pass a current greater than 2 amps.
In addition, since power equals the square of voltage divided by resistance, the voltage swing of the output devices in the power amplifier must develop twice the voltage into an 8 ohm load to create the same power level as into a 2 ohm load. This means that if an amplifier is rated to deliver a given power into either 2 or 8 ohms, the power supply must provide sufficient bias voltage on the amplifier's output devices to accommodate the 8 ohm rating-even though a 2 ohm load would require only half that bias voltage to produce the amplifier's rated power. Since power dissipated in the output devices equals current times voltage, it would be desirable to reduce output device bias voltage in the event the amplifier drives a low impedance load.